1. Field of the Invention
This invention relates to expanded beads of a base resin containing polypropylene resin and a process for the production thereof. The present invention also pertains to a foamed molding obtained from such expanded polypropylene-based resin beads.
2. Description of Prior Art
A polypropylene resin is now increasingly utilized in various fields because of excellent mechanical strength, heat resistance and recyclability thereof. Foamed moldings of a base resin including a polypropylene resin (such a propylene resin-containing base resin will be hereinafter sometimes referred to simply as “polypropylene-based resin”), which retain the above excellent properties of the polypropylene resin and which have excellent additional characteristics such as cushioning property and heat resistance, are thus utilized for various applications such as packaging materials, construction materials, etc. In particular, foamed moldings obtained by heating expanded beads of a base resin including a polypropylene resin in a mold with steam (such foamed moldings will be hereinafter sometimes referred to simply as “PP moldings”) are utilized as various parts such as bumper cores and door parts of automobiles.
Recently, there is an increasing demand for PP moldings having higher rigidity and lighter weight than the conventional ones in view of severe standard for collision safety and saving of energy consumption. Generally, however, the melting point of a polypropylene resin increases with an increase of the rigidity. In order to obtain PP moldings having high rigidity, therefore, it is necessary to use a high steam pressure in a large amount and to use a special molding device having a high pressure resistance.
To cope with this problem, International Publication WO 02/24794 proposes a process for the preparation of expanded polypropylene-based resin beads, in which particles of a base resin including a polypropylene resin are dispersed in a dispersing medium containing an organic peroxide. The dispersion is heated to decompose the organic peroxide, thereby obtaining organic peroxide-treated resin particles. The treated resin particles are foamed and expanded using a blowing agent to obtain the expanded polypropylene-based resin beads which permit the production of PP moldings using relatively low temperature steam.
There is also an increasing demand for PP moldings colored with a coloring agent. Japanese Kokai Publication No. H07-300537 discloses a method in which a polypropylene-based resin is previously kneaded with a coloring agent to obtain a master batch. A portion of the master batch and the polypropylene-based resin are then kneaded and formed into particles. The particles are then foamed and expanded using a blowing agent to obtain expanded polypropylene-based resin beads useful for producing colored PP moldings.